In the last few years, liquid crystal displays (LCDs) have jumped from being a small player to a dominant force in displays. First seen as monitors for computers, LCDs have advantages over cathode ray tube (CRT) displays. LCDs have a much smaller thickness, as they do not need the space or equipment to support the electron gun used in a CRT. This means that LCD displays could be used in places where a CRT was too bulky to fit neatly. The omission of the electron gun also lightened the display, meaning that an LCD is considerably lighter than a comparably sized CRT.
LCDs also have some disadvantages. The first disadvantage that most people think of is cost. An LCD usually costs more than a comparably sized CRT monitor. But as manufacturing has scaled up, cost is becoming less of an issue.
A second disadvantage of early model LCDs is their viewing angle. Whereas CRTs can be viewed from almost any angle that provides a line of sight with the screen, LCDs tend to have narrower viewing angles. If an LCD is viewed from outside its ordinary viewing angle, even if the screen is in a direct line of sight, the screen is essentially unreadable. Manufacturing has begun to address this problem, and LCDs today have viewing angles that are almost as good as CRT displays.
A third disadvantage is pixel responsiveness. In a CRT display, the electron gun generates an electron stream, which is directed to each pixel in turn. The pixels (actually a combination of three differently colored dots: usually one each of red, green, and blue) respond: the phosphors show the desired color. The time it takes for each pixel to respond to the electron stream is very small: typically less than 12 milliseconds (ms). And because the pixels begin to lose their color fairly quickly after being energized by the electron stream, the electron gun paints the entire surface of the display roughly 30 times per second. All this means that pixels in a CRT display respond very quickly to changes.
LCDs, in contrast, rely on polarized light to operate. Two polarized filters sandwich pixels. The two polarized filters are at 90° to each other. Because polarized filters block all light that is not at the correct angle, without the operation of the pixel, all light would be blocked. In its normal state, the pixel includes layers of molecules that twist the light 90°, so that light leaves the pixel oriented correctly relative to the second polarized filter. To change the amount of light passing through the pixel, a current is applied. The current untwists the pixel, meaning that light leaves the pixel at the same angle it had upon entering the pixel, and the second polarized filter blocks the light from being visible. But compared with CRTs, pixels in LCDs respond slowly: average response time is around 20 ms.
When used as computer monitors, the slow response time of LCDs is not a significant impediment, because typical computer use does not require pixels to change quickly. But as LCDs have begun to be used for video (e.g., to display digital video discs (DVDs) or as televisions), the slow response time of LCDs produces a noticeable effect. Images are blurred, especially where the pixels have to change values quickly (e.g., when there is fast action on the display).
Aware of this problem, manufacturers have attempted to improve pixel responsiveness by focusing on the materials. Changes to the liquid used in the liquid crystals can help to some extent. But there are limits to the responsiveness of any material used, and more advanced materials are also more expensive to manufacture.
Accordingly, a need remains for a way to improve pixel performance in LCDs without resorting to different materials that addresses these and other problems associated with the prior art.